1. Field of the Invention
The present invention relates to a molten metal casting process capable of producing castings which have few defects.
2. Description of the Related Art
As shown in FIG. 29, a casting mold 1 according to the conventional and basic operating system comprises a bottom mold section 2, a main mold section 3 mounted on the bottom mold section 2, and a top mold section 4 mounted on the main block 3. A sprue 7 and a runner 8 are formed in them. In addition, a cavity 5 shaped to match the shape of castings is formed in the main mold section 3 and the runner 8 is communicated with the cavity 5 through a gate 9. The runner 8 is also communicated with the sprue 7, which is further communicated with a pouring basin 6.
When molten metal is poured into the pouring basin 6 of this casting mold 1, it flows from the pouring basin 6 into the cavity 5, passing through the sprue 7, the runner 8 and the gate 9. In short, it flows through a molten metal passage. A riser or feeder (not shown) is usually arranged in the cavity 5. In addition, a stopper (not shown) and a sprue well 7c are sometimes arranged in the molten metal passage to control the flow of molten metal. Molten metal purifying units (not shown) such as a slag separator and a filter are also sometimes arranged in the molten metal passage. Fundamentally, however, the casting mold 1 has the above-mentioned molten metal passage.
The conventional running system, however, has the following problems (1) and (2).
1) In the initial molten metal pouring stage, molten metal rushes into the sprue 7 to thereby cause violent turbulence in the sprue well 7c, the runner 8 and that area in the cavity 5 which is adjacent to the gate 9. As a result, the molten metal is oxidized, and atmospheric gas is entrapped into the molten metal. This problem is quite old and various improvements have been proposed to solve it. In the fifties and sixties, studies were vigorously made to optimize the shape of the sprue 7 and to determine other measures at the same time. One of them was to arrange a stopper at a top 7a of the sprue 7 to control the amount of molten metal flowing during the initial molten metal pouring stage, while changing the shape of the sprue. It was confirmed, however, by X-ray viewing and other methods conducted at the casting time that the problem could not be completely solved by this proposal. Now, therefore, a recess such as the sprue well 7c is formed at a sprue exit 7b of the sprue 7. The impact of flowing molten metal is thus softened by the sprue well 7c to reduce the turbulence at the initial molten metal pouring stage.
2) In the common casting process, optimum casting speeds (including the speed of molten metal flowing into the cavity) are set experientially or by considering the surface tension of molten metal and the speed thereof. When the speed of flowing molten metal becomes higher than 0.5 m per second in the casting of molten aluminum, for example, the momentum of molten aluminum cannot be restrained by the surface tension thereof. The result is that the oxide film on the surface of moving molten aluminum meniscus breaks allowing further oxidation of the molten aluminum.
In the gravity casting process (which is the easiest and least expensive casting process particularly when an optimum-designed casting mold as shown in FIG. 2 is used), the difference H.sub.0 between heads of molten metal in the pouring basin and in the cavity becomes gradually smaller, gradually slowing the speed of molten metal as the casting process advances. It is therefore difficult, in this gravity casting process, to maintain the optimum casting conditions from the beginning of molten metal pouring stage to the end thereof. It is also quite difficult particularly in a large-sized casting to control the speed of flowing molten metal. Specific measures such as vacuum-assisted casting have been proposed to solve these drawbacks but they cannot become common when their equipment cost, their running manner, and their limit to the large-sized casting are considered.